The invention relates generally to semiconductor fabrication and more specifically to in-line metrology for process control during wafer processing.
During semiconductor fabrication a there are multiple steps where an underlying substrate is subjected to the formation and removal of various layers. The small feature sizes, tight surface planarity requirements, combined with the constant quest to increase throughput, makes it highly desirable to stop the process when the correct thickness has been achieved, i.e., when an endpoint has been obtained for the process step.
Eddy current sensors are used for displacement, proximity and film thickness measurements. The sensors rely on the induction of current in a sample by the fluctuating electromagnetic field of a test coil proximate to the object being measured. Fluctuating electromagnetic fields are created as a result of passing an alternating current through the coil. The fluctuating electromagnetic fields induce eddy currents which perturb the applied field and change the coils inductance.
FIG. 1 is a simplified schematic diagram of the principle upon which an eddy current sensor operates. An alternating current flows through coil 100 in close proximity to conducting object 102. The electromagnetic field of the coil induces eddy currents 104 in conducting object 102. The magnitude and the phase of the eddy currents in turn effect the loading on the coil. Thus, the impedance of the coil is impacted by the eddy currents. This impact is measured to sense the proximity of conducting object 102 as well as a thickness of the object. Distance 106 impacts the effect of eddy currents 104 on coil 100, therefore, if object 1002 moves, the signal from the sensor monitoring the impact of eddy currents on coil 100 will also change.
Attempts to use eddy current sensors to measure thickness of a film has resulted in limited success. Since the signal from the eddy current sensor is sensitive to both the thickness of the film and distance of the substrate to the sensor, there are two unknowns that must be resolved. FIG. 2 is a schematic diagram of a wafer carrier having an eddy current sensor for measuring the thickness of a wafer during a chemical mechanical planarization process (CMP). Wafer carrier 108 includes eddy current sensor 110. During a CMP operation, wafer 114 supported by carrier film 112 of carrier 108 is pressed against pad 116 to planarize a surface of the wafer. Pad 116 is supported by stainless steel backing 118.
One shortcoming of the configuration of FIG. 2 comes from the variability of the carrier film, which can vary by +/−3 mils. Thus, the carrier film causes a substantial variability in the distance between the wafer and the sensor. Additionally, different down forces applied to the carrier film will cause further variation as the carrier film compresses. Accordingly, it becomes extremely difficult to calibrate for all the variables that effect the distance, which in turn impacts the thickness measurement of the sensor. Another shortcoming of this configuration is caused by the presence of another conducting material separate from the conducting material being measured and is commonly referred to as a third body effect. If the thickness of the conductive layer is less than the so-called skin depth, the electromagnetic field from the coil will not be completely absorbed and will partially pass through to stainless steel backing 118 of pad 116 of FIG. 2. It will induce additional eddy currents within the stainless steel belt, thereby contributing to the total signal from the eddy current sensor. Furthermore, it should be appreciated that the pad wears or erodes over time, causing variation in the distance between the stainless steel backing and the eddy current sensor, which influences the appropriated contribution to the total eddy current sensor signal. Thus, a wear factor has to be considered as the wafers are continuously being processed. Consequently, due to the variability injected into the thickness measurement, the amount of error is unacceptably high and unpredictable.
In view of the foregoing, there is a need to eliminate or offset the variability inherent under working conditions so that an accurate endpoint can be determined to more precisely achieve a desired thickness.